PEP 517 get_requires_for_build_wheel hook attempts to compile CFFI modules

[hroncok]

Hello. In Fedora, we use the PEP 517 get_requires_for_build_wheel hook to gather the build dependencies.

With a project that uses setuptools.setup(cffi_modules=...), calling such hooks attempts to build the CFFI modules. That seems unexpected, as calling get_requires_for_build_wheel should not compile stuff.

To reproduce, I use the https://github.com/anrieff/libcpuid project at the current master branch:

$ git clone https://github.com/anrieff/libcpuid
$ cd libcpuid
[libcpuid (master)]$ uv venv --python=python3.13 venv
[libcpuid (master)]$ . venv/bin/activate
(venv) [libcpuid (master)]$ uv pip install pyproject_hooks setuptools cffi
 + cffi==1.17.1
 + pycparser==2.22
 + pyproject-hooks==1.2.0
 + setuptools==80.9.0
(venv) [libcpuid (master)]$ cd python/
(venv) [python (master)]$ LANG=C.utf-8 python
Python 3.13.7 (main, Aug 14 2025, 00:00:00) [GCC 14.3.1 20250523 (Red Hat 14.3.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from pyproject_hooks import BuildBackendHookCaller
>>> hook_caller = BuildBackendHookCaller('.', 'setuptools.build_meta')
>>> hook_caller.get_requires_for_build_wheel()
/usr/bin/ld: cannot find -lcpuid: No such file or directory
collect2: error: ld returned 1 exit status
Traceback (most recent call last):
  File ".../libcpuid/venv/lib64/python3.13/site-packages/pyproject_hooks/_in_process/_in_process.py", line 389, in <module>
    main()
    ~~~~^^
  File ".../libcpuid/venv/lib64/python3.13/site-packages/pyproject_hooks/_in_process/_in_process.py", line 373, in main
    json_out["return_val"] = hook(**hook_input["kwargs"])
                             ~~~~^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../libcpuid/venv/lib64/python3.13/site-packages/pyproject_hooks/_in_process/_in_process.py", line 143, in get_requires_for_build_wheel
    return hook(config_settings)
  File ".../libcpuid/venv/lib64/python3.13/site-packages/setuptools/build_meta.py", line 331, in get_requires_for_build_wheel
    return self._get_build_requires(config_settings, requirements=[])
           ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../libcpuid/venv/lib64/python3.13/site-packages/setuptools/build_meta.py", line 301, in _get_build_requires
    self.run_setup()
    ~~~~~~~~~~~~~~^^
  File ".../libcpuid/venv/lib64/python3.13/site-packages/setuptools/build_meta.py", line 317, in run_setup
    exec(code, locals())
    ~~~~^^^^^^^^^^^^^^^^
  File "<string>", line 3, in <module>
  File ".../libcpuid/venv/lib64/python3.13/site-packages/setuptools/__init__.py", line 115, in setup
    return distutils.core.setup(**attrs)
           ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File ".../libcpuid/venv/lib64/python3.13/site-packages/setuptools/_distutils/core.py", line 148, in setup
    _setup_distribution = dist = klass(attrs)
                                 ~~~~~^^^^^^^
  File ".../libcpuid/venv/lib64/python3.13/site-packages/setuptools/dist.py", line 321, in __init__
    _Distribution.__init__(self, dist_attrs)
    ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
  File ".../libcpuid/venv/lib64/python3.13/site-packages/setuptools/_distutils/dist.py", line 309, in __init__
    self.finalize_options()
    ~~~~~~~~~~~~~~~~~~~~~^^
  File ".../libcpuid/venv/lib64/python3.13/site-packages/setuptools/dist.py", line 784, in finalize_options
    ep(self)
    ~~^^^^^^
  File ".../libcpuid/venv/lib64/python3.13/site-packages/setuptools/dist.py", line 804, in _finalize_setup_keywords
    ep.load()(self, ep.name, value)
    ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
  File ".../libcpuid/venv/lib64/python3.13/site-packages/cffi/setuptools_ext.py", line 216, in cffi_modules
    add_cffi_module(dist, cffi_module)
    ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
  File ".../libcpuid/venv/lib64/python3.13/site-packages/cffi/setuptools_ext.py", line 49, in add_cffi_module
    execfile(build_file_name, mod_vars)
    ~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../libcpuid/venv/lib64/python3.13/site-packages/cffi/setuptools_ext.py", line 25, in execfile
    exec(code, glob, glob)
    ~~~~^^^^^^^^^^^^^^^^^^
  File "src/libcpuid/_ffi_build.py", line 90, in <module>
    NO_SIZEOF_HEADER = eval_sizeofs(PREPROCESSED_HEADER, EVAL_SIZEOF_CFLAGS)
  File "src/libcpuid/_ffi_build.py", line 62, in eval_sizeofs
    subprocess.check_call(["gcc", c_program_path, *cflags, "-o", executable_path])
    ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.13/subprocess.py", line 419, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['gcc', PosixPath('/tmp/tmpvmg_lwo9/sizeof.c'), '-I.../libcpuid/libcpuid', '-L.../libcpuid/libcpuid/.libs', '-lcpuid', '-Wl,-rpath=.../libcpuid/libcpuid/.libs', '-o', PosixPath('/tmp/tmpvmg_lwo9/sizeof')]' returned non-zero exit status 1.
Traceback (most recent call last):
  File "<python-input-2>", line 1, in <module>
    hook_caller.get_requires_for_build_wheel()
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File ".../libcpuid/venv/lib64/python3.13/site-packages/pyproject_hooks/_impl.py", line 196, in get_requires_for_build_wheel
    return self._call_hook(
           ~~~~~~~~~~~~~~~^
        "get_requires_for_build_wheel", {"config_settings": config_settings}
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File ".../libcpuid/venv/lib64/python3.13/site-packages/pyproject_hooks/_impl.py", line 392, in _call_hook
    self._subprocess_runner(
    ~~~~~~~~~~~~~~~~~~~~~~~^
        [python, abspath(str(script)), hook_name, td],
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        cwd=self.source_dir,
        ^^^^^^^^^^^^^^^^^^^^
        extra_environ=extra_environ,
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File ".../libcpuid/venv/lib64/python3.13/site-packages/pyproject_hooks/_impl.py", line 83, in default_subprocess_runner
    check_call(cmd, cwd=cwd, env=env)
    ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.13/subprocess.py", line 419, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['.../libcpuid/venv/bin/python', '.../libcpuid/venv/lib64/python3.13/site-packages/pyproject_hooks/_in_process/_in_process.py', 'get_requires_for_build_wheel', '/tmp/tmpvtcdk7vc']' returned non-zero exit status 1.

Expected results: ['setuptools, 'cffi']

Note that in Fedora, we don't have -lcpuid yet at this point because the library is built as part of the same Fedora package, and when we gather the dependencies, we haven't yet built the library. But that's not the point. The point is, building any code with gcc during get_requires_for_build_wheel or prepare_metadata_for_build_wheel hooks is unexpected.

cc @zacikpa @TheTumultuousUnicornOfDarkness @yselkowitz

[zacikpa]

This is my doing after all. Python-CFFI does not support sizeof in FFI().cdef() so there is a nasty workaround in https://github.com/anrieff/libcpuid/blob/master/python/src/libcpuid/_ffi_build.py where I replace each sizeof in the libcpuid header file manually by running a piece of C code that evaluates it.

[hroncok]

But why is that code even executed?

[zacikpa]

It's run to determine the size of structs from the libcpuid library, which I need to know to preprocess the header file into a sizeof-free format. Does that answer your question?

[hroncok]

I mean, when calling the get_requires_for_build_wheel hoook. That codepath should be avoided, at least that's my expectation.

[zacikpa]

Looks like setuptools executes the setup script here (collecting requirements by attempting to run the setup script and checking for errors IIUC).